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Method for transmitting telegrams between a control device and a peripheral element via an intermediate device

Method for transmitting telegrams between a control device and a peripheral element via an intermediate device description/claims

This application claims priority of European application No. 07015896.9 filed Aug. 13, 2007, which is incorporated by reference herein in its entirety.

The present invention relates to a method for transmitting telegrams between a control device and a peripheral element.

In automation technology, the safe control of machines and equipment is imperative. In particular safety-oriented information must be reliably transmitted between the control device and the peripheral element. Furthermore, safe processing must be guaranteed. In this case the expression “safe processing” means that a single error must have no dangerous consequences. The expression “reliable transmission” means that transmission errors can be excluded with reasonable certainty. As an example of safe processing, two control devices monitor one another and arrive at a safe state in a controlled system as soon as one of the two control devices detects an error state. A further example of safe processing is when a control device considered to be unreliable per se and a peripheral element considered to be unreliable per se are monitoring one another and bring about a suitable safety reaction in the event of an error being detected.

Reliable transmission requires on the one hand that errors in telegrams from the respective receiving unit can be detected. It requires furthermore that above and beyond telegram errors as such, the sequence and non-arrival of telegrams can also be reliably detected.

It is known from the prior art that data can be transmitted between the control device and the peripheral element over separate lines. It is further known that the telegrams can be transmitted over a bus from the control device to the peripheral element or vice versa. In this case, in the prior art the control device and the peripheral element are on the one hand connected to the bus. On the other, the telegrams are safety telegrams, so that telegrams transmitted to the control device can be checked for freedom from errors by said control device, and telegrams transmitted to the peripheral element can be checked for freedom from errors by said peripheral element.

Automation tasks are often carried out by hierarchically structured automation systems. If in such a case further components (=intermediate device within the meaning of the present invention) are arranged between the control device and the peripheral element, as a rule the information relating to safety is transmitted over dedicated lines via which the respective peripheral element is connected to the control device.

Document DE 199 28 517 A1 discloses a control system that has a first reliable control device, a second, unreliable control device and peripheral devices. The reliable control device is arranged between the unreliable control device and the peripheral devices. The reliable control device is in a position to receive telegrams from the second, unreliable control device and to forward said telegrams either amended or without amendment to the peripheral devices. The reliable control device is furthermore in a position to forward telegrams received from the peripheral devices without amendment to the second, unreliable control device and at the same time to monitor said telegrams.

Document WO 2006/029899 A discloses a method for transmitting telegrams between a control device and a peripheral element.

The object of the present invention is to create options whereby safety-oriented telegrams can be transmitted via the intermediate device, even though the intermediate device performs further tasks, in particular time-critical tasks.

This object is achieved by the claims.

According to the invention, in a method for the transmission of telegrams between a control device and a peripheral element for transmitting one of the telegrams from the control device to the peripheral element, an intermediate device receives the respective telegram from the control device, buffers it, and at the latest after a maximum buffer time forwards it without amendment to the peripheral element. A like method applies to the transmission of one of the telegrams from the peripheral element to the control device. Here also the intermediate device receives the respective telegram, buffers it, and at the latest after a maximum buffer time forwards it without amendment. The telegrams are safety telegrams, so that the telegrams from the respective receiving unit (control device or peripheral element) can be checked for freedom from errors. In this case, the check for freedom from errors includes on the one hand internal freedom from errors in the respective telegrams and on the other hand both the correct sequence and the non-arrival of telegrams.

The intermediate device receives and forwards telegrams in the context of a switching functionality. In addition to the switching functionality, the intermediate device has its own dedicated functionality. Said intermediate device is designed so that a write access to the buffer (in which the received telegrams are temporarily stored) is locked out in the context of the dedicated functionality. This applies even when, in the context of said dedicated functionality, an attempt is made to amend telegrams. This method makes it impossible for telegrams to be corrupted by the dedicated functionality.

Preferably the intermediate device receives the respective telegram over a logical input channel, determines a logical output channel corresponding to the logical input channel, and forwards the respective telegram over said logical output channel. This method has the advantage that the intermediate device can handle the communication in a particularly simple manner.

Preferably the intermediate device determines the logical output channel by means of an internal configuration. This is a particularly flexible method.

As a rule the telegrams exchanged between the control device and the peripheral element refer to one safety function each. Preferably each safety function is individually assigned to one logical input channel and one corresponding logical output channel per direction of transmission. By this method the individual safety functions can be handled separately from one another.

Unlike a write access, a read access to the buffer is not critical. The intermediate device can therefore be designed so that a read access to the buffer is possible in the context of the dedicated functionality. In particular, safety-related information exchanged between the control device and the peripheral device can thereby be taken into account in the context of the dedicated functionality.

The admissibility of access to the buffer in the context of the dedicated functionality is preferably determined by an internal configuration of the intermediate device, definable independently of the dedicated functionality of said intermediate device. In this way it is a simple matter to ensure that the buffer is closed to a write access but open (if necessary only partly) to a read access.

It is possible for the intermediate device to have a program memory in which a program is stored, said program being executable by a corresponding device in the intermediate device and the execution of said program bringing about the inventive operation of the intermediate device. Alternatively the intermediate device can have an ASIC for this purpose.

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